Semiconductor elements, and diaphragm sensors in particular, are known already, as well as methods for producing diaphragm sensors on the base of semiconductor substrates such as silicon wafers. Flat porous diaphragm regions are arranged on the semiconductor substrate as substrate layer for sensor structures, for instance, and a cavity is produced underneath the diaphragm for, e.g., the thermal insulation of the diaphragm.
The diaphragm sensors currently on the market are mostly fashioned as thin film diaphragm sensors. For this purpose, layer systems in thicknesses between a few 10 nm and a few μm are deposited on a carrier substrate and after that, the carrier substrate is removed in predefined areas, so as to obtain self-supporting areas. The structural elements of the sensor can then be arranged in the center of the diaphragm.
Surface micromechanics (SMM), in which a sacrificial layer is generally used that is deposited on the front side of a carrier substrate before diaphragm deposition, constitute another possibility for exposing the diaphragm. The sacrificial layer is later removed from the component side of the sensor through “detachment openings” in the diaphragm, whereby a self-supporting structure is created. These surface micromechanical methods are comparatively costly, on account of the necessity for separate sacrificial layers.
Published German patent document DE 100 32 579 discloses a method for manufacturing a semiconductor element and also a semiconductor element produced according to the method, in which, e.g., for a diaphragm sensor, a layer of semiconductor substrate material that was rendered porous is situated above a cavity. Two layers having different porosity are formed to produce the cavity using appropriate etching parameters. Whereas the first layer has lower porosity and seals up during a subsequent first annealing step, the porosity of the second layer increases during the annealing step in such a way that a cavity or a cavern is formed. In a second process step, at a higher annealing temperature, a relatively thick epitaxial layer as second diaphragm layer is grown on top of the first diaphragm layer formed from the first porous layer.
It may also be provided that a thin epitaxial layer be grown during the first annealing step so as to ensure complete sealing of the porous first layer, which is used as starting layer for the epitaxy growth of the thick epitaxial layer. In this context, a lower growth rate is preferred at a lower temperature compared to the subsequent deposition of the thick epitaxial layer.
As a result of the measures mentioned, the construction of an SMM semiconductor element can be simplified considerably since no additionally deposited sacrificial layer is required and, furthermore, the diaphragm itself or an essential portion of the diaphragm is produced from semiconductor substrate material.
However, tests have shown that at least partially porous diaphragm may get damaged already during production, or that damage may not always be reliably prevented under normal operating conditions. To avoid damage to the diaphragm during manufacture or in regularly occurring application cases, published German patent document DE 101 38 759 provides a method for manufacturing a semiconductor element having a semiconductor substrate, in which the semiconductor substrate in the region of the porous diaphragm layer receives a doping that differs from the doping in the region of the later cavity. After doping, the semiconductor material of the diaphragm layer is rendered porous, and the semiconductor material underneath the semiconductor material that has been rendered porous is at least partially removed or rearranged to provide a cavity.
Published German patent document DE 100 30 352 discloses a micromechanical component which has a support body made of silicon and a regionally self-supporting diaphragm which is joined to the support body. The diaphragm is regionally and superficially provided with at least one stabilizing element for support. To form the regionally self-supporting diaphragm, it is provided that the silicon is rendered porous in a first region and is selectively removed via an etching opening once the diaphragm layer has been deposited.